Most electronic devices today include a large number of electronic components or integrated circuits (ICs) which are mounted to circuit boards which are arranged within the device. The components are attached to the boards and each of the components has a large number of electrical leads which must be individually connected to specific electrical points on the circuit board. The components are held to the board with adhesive materials and also, at least in part, by the soldered electrical connections.
In order to increase the number of components than can be mounted within a given device volume and therefore decrease the size of, or miniaturize, many electronic devices, circuit boards are being assembled having components mounted on both sides of a circuit board instead of having components mounted on only one side of the board. During the assembly process, circuit board components are mounted on a first side of the circuit board during a first pass-through of a soldering oven during which a mixture of solder beads and flux between the components and the circuit board are heated to a point at which the solder begins to flow as a liquid. As the circuit board completes the oven process, the solder cools and hardens thereby forming a solid electrical and mechanical connection between the component connection points and the corresponding connection points on the circuit board. The components are mounted on the second side of the circuit board by inverting the board and running a second pass of the board through the soldering oven during which components are mounted on the second side of the circuit board.
During the second pass through the solder oven, the components which were mounted during the first pass are on the bottom side of the board and are subject to the force of gravity which asserts a force tending to pull the components downwardly and away from the board. As the first side components pass the "reflow" area of the solder oven, the soldered connections made during the first pass change state from solid to liquid or "reflow" and reduce the surface tension which holds the components to the circuit boards. When the ratio of the mass of the components to the surface tension holding the components to the boards exceeds a certain value, the first pass soldered connections will become weak and there is a tendency for the components mounted during the first pass to fall off the underside of the board and the board will have to be reworked.
The industry's current emphasis on miniaturization and environmentally-friendly soldering (inert gas blanket, no-clean flux and low solid flux) increases the component retention problem because the trends are to reduce the surface area and the surface tension of the solder. While some new techniques aid the soldering process during a first pass, such techniques also increase the fall-off rate for components going through a second pass on the underside of a circuit board.
Thus, there is a need for an improved methodology and implementing system for mounting electronic components on printed circuit boards and controlling solder reflow.